A sampling rate conversion apparatus is used to convert a digital signal with a first sampling rate (or a first sampling frequency) into a digital signal with a second sampling rate (or a second sampling frequency). Exemplary sampling rate conversion techniques are disclosed in U.S. Pat. Nos. 6,509,850 and 5,913,190, U.S. Published Application No. 2002/0046227 and Published Korean Application No. 2001-0112790.
FIG. 1 is a block diagram schematically showing a conventional audio reproducing system that includes sampling rate conversion. Referring to FIG. 1, the conventional audio reproducing system 100 includes an audio storage unit 110, an audio signal input unit 120, a processor 130, a digital-to-analog converter (DAC) 150, and an audio signal output unit 160. The conventional audio reproducing system 100 may be a karaoke, a CD (Compact Disc) player, a DVD (Digital Versatile Disc) player and/or an MP3 (MPEG layer 3) player.
The audio signal storage unit 110 can include a storage medium such as a CD or a DVD. The audio signal storage unit 110 supplies a second digital audio stream (or a digital audio signal) AUD12 to the processor 130. For example, a sampling rate of a digital audio stream output from a CD is 44.1 kHz and a sampling rate of a digital audio signal output from a DVD is 48 kHz.
The audio signal input unit 120 may include a digital audio input unit such as a microphone. The audio signal input unit 120 supplies a first digital audio stream AUD11 to the processor 130. For example, a sampling rate of a digital audio stream output from the microphone may be 8 kHz.
The processor 130 operates as a controller and includes a mixer 131, a sampling rate detector 132, and a resampler 140. The sampling rate detector 132 and the resampler 140 can act as a sampling rate conversion apparatus.
The sampling rate conversion apparatus converts a sampling rate of the first digital audio stream (hereinafter referred to as an input stream) AUD11 and generates a third digital audio stream (hereinafter referred to as an output stream) AUD13 with the same sampling rate as the second digital audio stream AUD12. The sampling rate of the first digital audio stream AUD11 may be referred to as a current sampling rate of the first digital audio stream AUD11. Moreover, the sampling rate of the second digital audio stream AUD12 may be referred to as a target sampling rate to be converted.
The mixer 131 mixes the second digital audio stream AUD12 with the output stream AUD 13. The mixer 131 is included in the processor 130 in FIG. 1, but the mixer 131 can be located outside the processor 130.
The DAC 150 converts a digital audio signal output from the mixer 131 to an analog audio signal and supplies the analog audio signal to the audio signal output unit 160 for reproducing an audio signal. The audio signal output unit 160 may be a speaker.
The sampling rate detector 132 detects the sampling rate of the second digital audio stream AUD12 and the sampling rate of the input stream AUD11, and supplies respective integer factors SR12 and SR11, corresponding to the respective sampling rates, to a respective upsampler 141 and a downsampler 143 included in the resampler 140. If the input stream AUD11, is an output signal of a microphone and the second digital audio stream AUD12 is an output signal of a CD, the second integer factor SR12 can be 441 and the first integer factor SR11 can be 80.
The resampler 140 includes the upsampler 141, a low-pass filter (LPF) 142 and the downsampler 143. The upsampler 141 increases the sampling rate of the input stream AUD11 by a multiple of the second integer factor SR12 in response to the second integer factor SR12. If the input stream AUD11 is an output signal of a microphone and the second digital audio stream AUD12 is an output signal of a CD, then 8 kHz as the sampling rate of the input stream AUD11 is 441-fold (SR12) increased by the upsampler 141. That is, the upsampler 141 increases the sampling rate of the input stream AUD11 up to a sampling rate corresponding to a least common multiple (that is, 8000×441=3528 (kHz)) of the sampling rates of the input stream AUD11 and the second audio stream AUD12.
The LPF 142 performs low-pass filtering of the input stream AUD11 with the increased sampling rate to thereby cancel or reduce sources of aliasing, in order to prevent or reduce aliasing.
The down sampler 143 decreases the sampling rate of the input stream AUD11 by a multiple of the first integer factor SR11 in response to the first integer factor SR11. For example, if the input stream AUD11 is an output signal of a microphone and the second digital audio stream AUD12 is an output signal of a CD, then the downsampler 143 decreases, by 80 fold, 3528 kHz (that is, 8000×441=3528 (kHz)) which is the sampling rate of the input stream increased by the upsampler 141 and generates an output stream AUD13 with a sampling rate of 44100 Hz.